1. Field of the Invention
The present invention relates to the transmission field and in particular concerns the radio transmissions based upon multi-carrier modulation techniques. Still more in particular it is concerned with a new technique for reducing the peak-to-average power ratio in such radio transmission systems.
2. Description of the Prior Art
Recently, the multi-carrier modulation is becoming more and more attracting in the field of the modulation schemes thanks to its inherent robustness characteristics in frequency-selective channels with a high fading and in mobile radio channels. This and other reasons have made the multi-carrier modulation, in all its variants, the most appropriate one for systems such as DAB and DVB-T, ADSL, and wireless LAN, and for OFDMA access systems.
One of the problems affecting every multi-carrier systems, and which often discourages the use thereof, arises with the non-constant signal envelope through the time, and in particular with the possible amplitude peaks that could be much higher than the average values exhibited thereby; the signal is said to have a high Peak-to-Average Power Ratio (PAR). This hinders the use of high-efficiency amplification devices which exhibit deep nonlinearities, otherwise intermodulation products that distort the in-band signal and increase the out-of-band spectral content thereof will be generated, thus causing a greater interference with adjacent channels and degrading the error probability at reception side.
In order to reduce the effects of such a phenomenon, conventionally the simple reduction of power amplifier operating point has been performed, resulting in the consequent efficiency loss; this, when the transmitted power is maintained constant, results in an increase of power consumption and device cost. The increase in the out-of-band radiation is also present should signal predistortion techniques be employed, which techniques allow to compare the amplifier behavior with the one of an ideal clipper.
There are several techniques that are able to reduce PAR in OFDM transmitters; their efficiency and realization complexity vary a lot according to the circumstances. In general, the most effective techniques ([D1]: J. S. Chow, J. A. C. Bingham, M. S. Flowers, “Mitigating Clipping Noise in Multi-carrier Systems”, IEEE Proc. int. Conf. Comm., Montreal Canada, pages 715-719, June 1997; [D2]: S. H. Muller, R. W. Bauml, R. F. H. Fischer, J. B. Huber, “OFDM with Reduced Peak-to-Average Power Ratio by Multiple Signal representation”, Annals of Telecommunications, Vol. 52, No. 1-2, pages 58-67, February 1997) involve a significant complexity increase, especially at the transmitter. Conversely, the simpler ones ([D3]: S. H. Muller, J. B. Huber, “A Novel Peak Power Reduction Scheme for OFDM”, Proc. Int. Symp. On Pers., Ind. and Mob. radio Comm. PIMRC '97, Helsinki, Finland, pages 1090-1094, September 1997; [4]: J. Tellado, J. M. Cioffi, “Peak Power Reduction for Multi-carrier Modulation”, Proc. IEEE Globecom, Commun. Theory Mini Conf. (CTMC), Sydney, Australia, November 1998) have a limited effectiveness or result in a performance degradation that is not always deemed acceptable. Moreover, the implementability of each of them depends greatly on the specific application (type of channel, technology of the devices, presence of a return channel) as well as on the fundamental OFDM system parameters (number of carriers, Base Band or passband).